The channel coding part is a very important component especially in wireless communication systems like UMTS, WLAN and WPAN. An efficient method of channel coding in packet based systems such as wireless communication systems is an Automatic Repeat Request (ARQ) system. By combining pure repetition with channel coding (FEC) a more robust system called a hybrid ARQ is constructed. These hybrid ARQ (HARQ) systems are mainly used in the wireless domain.
In wireless communication systems, hybrid ARQ schemes are used to guarantee an error free transmission of data packets. In contrast to the traditional ARQ scheme where no channel coding is used, a hybrid ARQ scheme uses a FEC (Forward Error Correction) code.
Three different kinds of hybrid ARQ schemes exist. In Hybrid ARQ type I the received packets with errors are discarded and a new packet is requested. No combination is performed in the receiver. For the repeated packet a stronger code might be used.
In Hybrid ARQ type II the received packets with errors are stored and the same packet is requested again. The newly received packet is then combined in a soft way with the stored previous packet. The normal technique used here is the Chase Combining algorithm. Another hybrid ARQ type II technique is the incremental redundancy technique.
In some publications the incremental redundancy scheme is called hybrid ARQ type III. Here the additional packet contains additional information. This additional information is combined with the stored packet. The new combined packet is then processed by the channel decoder.
Different types of channel coding can be used. In wireless communications, convolutional codes, turbo-codes and Low Density Parity Check (LDPC) codes are examples of codes which can be used as a FEC code.
Especially in the case of hybrid ARQ type II and type III, the main challenge in the receiver is the management of the needed memory to store the soft values for the combination process. In the traditional approach, the soft values are combined before the deployment of the channel decoder. Thus the previously received packet (soft values) needs to be stored until the repeated packet arrives. After the reception of the new packet, the stored packet and the new packet need to be combined. The combined packet is then processed by the channel decoder.